1. Field of the Invention
This invention relates to stents, a type of implantable medical device with a prohealing coating, and an antiproliferative coating selectively applied to the abluminal surface.
2. Background
This invention relates to stents, which, among other uses, are used in the treatment of artherosclerosis. Atherosclerosis is a progressive disease which results in build-up of materials such as fats, cholesterol, calcium and cellular debris, the build-up collectively referred to as plaques, within the walls of arteries. The build-up of plaque along the artery walls results in hardening and constriction of the artery. When an artery that provides blood to the heart is clogged, resulting in a loss of blood flow or a severe reduction in blood flow to the heart, a heart attack results. A clot in an artery leading to the brain, potentially resulting from dislodged arterial plaque, results in a stroke. Coronary artery disease, the hardening and narrowing of arteries to the heart often the resulting from artherosclerosis, is the leading cause of death in the United States for both men and women.
This invention relates, more generally, to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. The stent may be visualized during delivery and deployment using X-Ray fluoroscopy, if it contains radio-opaque materials.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated, and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a constraining member, such as a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. In addition, the stent must possess sufficient flexibility to allow for crimping, expansion, and cyclic loading. Longitudinal flexibility is important to allow the stent to be maneuvered through a tortuous vascular path, and to enable it to conform to a deployment site that may not be linear, or may be subject to flexure. Finally, the stent should be biocompatible, so as not to trigger any adverse responses.
The structure of a stent is typically composed of scaffolding that includes a pattern, or network, of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed so that the stent can be radially compressed (to allow crimping) and radially expanded (to allow deployment). A conventional stent is allowed to expand and contract through movement of individual structural elements of a pattern with respect to each other.
Additionally, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier, or other carrier, that includes an active agent, bioactive agent, or drug. If the stent body includes a polymer, or the stent scaffolding is made from a polymer, the stent body may also serve as a carrier of an active agent or drug.
Furthermore, it may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended.